In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder or powder suspension referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly. In a slightly different arrangement, toner may be transferred to an intermediate surface, prior to retransfer to a final substrate.
Transfer of toner from the charge retentive surface to the final substrate is commonly accomplished electrostatically. A developed toner image is held on the charge retentive surface with electrostatic and mechanical forces. A substrate (such as a copy sheet) is brought into intimate contact with the surface, sandwiching the toner thereinbetween. An electrostatic transfer charging device, such as a corotron, applies a charge to the back side of the sheet, to attract the toner image to the sheet.
Unfortunately, the interface between the sheet and the charge retentive surface is not always optimal. Particularly with non-flat sheets, such as sheets that have already passed through a fixing operation such as heat and/or pressure fusing, or perforated sheets, or sheets that are brought into imperfect contact with the charge retentive surface, the contact between the sheet and the charge retentive surface may be non-uniform, characterized by gaps where contact has failed. There is a tendency for toner not to transfer across these gaps. A copy quality defect referred to as transfer deletion results.
The problem of transfer deletion has been unsatisfactorily addressed by mechanical devices that force the sheet into the required intimate and complete contact with the charge retentive surface. Blade arrangements that sweep over the back side of the sheet have been proposed, but tend to collect toner if the blade is not cammed away from the charge retentive surface during the interdocument period, or frequently cleaned. Biased roll transfer devices have been proposed, where the electrostatic transfer charging device is a biased roll member that maintains contact with the sheet and charge retentive surface. Again, however, the roll must be cleaned. Both arrangements can add cost, and mechanical complexity.
That acoustic agitation or vibration of a surface can enhance toner release therefrom is known, as described by U.S. Pat. No. 4,111,546 to Maret, U.S. Pat. No. 4,684,242 to Schultz, U.S. Pat. No. 4,007,982 to Stange, U.S. Pat. No. 4,121,947 to Hemphill, Xerox Disclosure Journal "Floating Diaphragm Vacuum Shoe, by Hull et al., Vol. 2, No. 6, November/December 1977, U.S. Pat. No. 3,653,758 to Trimmer et al., U.S. Pat. No. 4,546,722 to Toda et al., U.S. Pat. No. 4,794,878 to Connors et al., U.S. Pat. No. 4,833,503 to Snelling, Japanese Published Patent Application 62-195685, U.S. Pat. No. 3,854,974 to Sato et al., and French Patent No. 2,280,115.
Resonators for applying vibrational energy to some other member are known, for example in U.S. Pat. No. 4,363,992 to Holze, Jr. which shows a horn for a resonator, coupled with a piezoelectric transducer device supplying vibrational energy, and provided with slots partially through the horn for improving non uniform response along the tip of the horn. U.S. Pat. No. 3,113,225 to Kleesattel et al. describes an arrangement wherein an ultrasonic resonator is used for a variety of purposes, including aiding in coating paper, glossing or compacting paper and as friction free guides. U.S. Pat. No. 3,733,238 to Long el al. shows an ultrasonic welding device with a stepped horn. U.S. Pat. No. 3,713,987 to Low shows ultrasonic agitation of a surface, and subsequent vacuum removal of released matter.
Coupling of vibrational energy to a surface has been considered in Defensive Publication T893,001 by Fisler. U.S. Pat. No. 3,635,762 to Ott et al., U.S. Pat. No. 3,422,479 to Jeffee, U.S. Pat. No. 4,483,034 to Ensminger and U.S. Pat. No. 3,190,793 Starke.
In the ultrasonic welding horn art, as exemplified by U.S. Pat. No. 4,363,992 to Holze, Jr., where blade-type welding horns are used for applying high frequency energy to surfaces, it is known that the provision of slots through the horn perpendicular to the direction in which the welding horn extends, reduces undesirable mechanical coupling of effects across the contacting horn surface. Accordingly, in such art, the contacting portion of the horn is maintained as a continuous surface, the horn portion is segmented into a plurality of segments, and the horn platform, support and piezoelectric driver elements are maintained as continuous members. For uniformity purposes, it is desirable to segment the horn so that each segments acts individually. However, a unitary construction is also highly desirable, for fabrication and mounting purposes.
It has been noted that even with fully segmented horns, as shown in U.S. Pat. No. 5,025,291 to Nowak et al., there is a fall-off in response of the resonator at the outer edges of the device and generally, some segment to segment non-uniformity. A similar fall off is shown in U.S. Pat. No. 4,363,992 to Holze, Jr., at FIG. 2, showing the response of the resonator of FIG. 1.
Of interest is U.S. Pat. No. 4,833,503 to Snelling, which describes ultrasonic transducer-driven toner transport in a development system, in which a current source provides a wave pattern to move toner from a sump to a photoreceptor. U.S. Pat. No. 4,568,955 to Hosoya et al. teaches recording apparatus with a developing roller carrying developer to a recording electrode, and a signal source for propelling the developer from the developing roller to the recording media.
The key to uniform vibration amplitudes across an ultrasonic resonator of the type used to enhance and enable electrophotographic processes is the decoupling of desired axial resonator motion (motion perpendicular to the charge retentive surface that caused toner release towards the final substrate) from undesirable transverse motion (motion in the cross process direction, parallel to the charge retentive surface). Even when resonator design parameters are optimized, transverse segmentation and discrete voltage modifications (as in U.S. Pat. No. 5,010,369 to Nowak et al. and U.S. Pat. No. 5,025,291 to Nowak et al. and U.S. patent application Ser. No. 07/887,037 entitled, "Edge Effect Compensation in High Frequency Vibratory Energy Producing Devices for Electrophotographic Imaging" by W. Nowak) will not completely eliminate this cross process direction non-uniformity. The root problem of non-uniformity is shown in FIG. 1A-1C, which shows, at FIG. 1A, a segmented transducer design (with segmented horn). At FIG. 1B, the frequency response amplitude over a 5 KHz range of individual horn segments along the length of a resonator is shown, illustrating the respective responses in the axial direction (labeled) and the transverse direction (labeled). At FIG. 1C, a plot of peak response amplitude of individual segments at 64 KHz in a resonator having 32 segments is shown, with non-uniformity resulting from bending and axial mode cross coupling at the arrow-marked areas.
Because mechanical continuum behavior in one dimension effects behavior in other dimensions, physical decoupling of what is referred to as the "Poisson effect" is required, by segmenting the transducer, as shown in FIG. 1A, and described in U.S. Pat. No. 5,025,291 to Nowak et al. This minimizes, but alone cannot eliminate, the effect of the undesirable transverse modes along the length of the resonator, and maximizes axial transducer motion. Theoretically, a structure completely eliminating the transverse mode would provide discrete resonator segments. Such a structure is not practical, since the vibratory energy of the resonator must somehow be coupled across the entire process width of the charge retentive surface. Additionally, it is highly desirable to have a unitary assembly for manufacturing and service reasons. It is speculated by the present inventors that such discrete resonators could be coupled with a compliant bond between individual segments, or with a compliant segment holder, but horn tip alignment and structural instability would be a major concern, with horn tip motion during operation on the order of 1 micron. Thus, complete segmentation is not practical.
All the references cited herein are specifically incorporated by reference for their teachings.